In an engine that is constructed so as to change air-fuel ratios of an air-fuel mixture supplied thereto in accordance with operation states of the engine, it is a general practice to determine a target air-fuel ratio on the basis of an amount of intake air. A sensor to be used here for the purpose to detect amounts of intake air is likely to cause the hunting or the overshoot. In particular, a sensor of a flap type, which is equipped with a flap portion designed to be displaced by a flow of intake air, has a remarkable tendency to cause such phenomena. Accordingly, an intake air amount detecting sensor as intake air amount detecting means for detecting amounts of intake air is designed so as to subject outputs therefrom to a sort of smoothing treatment, thereby causing a sensitivity of the sensor to be substantially decreased. The smoothing treatment referred to here is to obtain an artificial amount of intake air on the basis of a current amount of intake air detected by the sensor and a past amount of intake air stored in advance in memory means and then determine a target air-fuel ratio on the basis of the artificial amount of intake air. This treatment serves as a prevention from adverse influences caused by the hunting or the like.
There is also a growing tendency that an engine uses the feedback control of an air-fuel ratio of the air-fuel mixture based on an air-fuel ratio of exhaust gases in order to achieve an accurate control of air-fuel ratios for an internal combustion engine. Japanese Patent Early Publication No. 32,946/1983 discloses a so-called "lean sensor" as an air-fuel ratio sensor for detecting air-fuel ratios of exhaust gases, which is designed to give outputs corresponding to air-fuel ratios of exhaust gases, thereby leading to a leaner air-fuel ratio of the air-fuel mixture, say, a larger air-fuel ratio.
It is to be understood that an amount of intake air corresponds eventually to an engine load, say, a throttle opening, and that a variation in amounts of intake air becomes larger with respect to a variation in the throttle openings when the throttle opening is still small, while a variation in amounts of intake air becomes smaller with respect thereto when the throttle opening gets larger. Accordingly, if a target air-fuel ratio is determined by dependence only upon the amounts of intake air, it becomes difficult to make an accurate detection of amounts of intake air particularly in operation states in which amounts of intake air get larger. In order to overcome the drawbacks encountered in the prior art, U.S. patent application Ser. No. 813,933, now U.S. Pat. No. 4,662,339, corresponding to Japanese Patent Early Publication No. 167,134/1986 proposes the determination of a target air-fuel ratio for an engine using a throttle opening as well as an amount of intake air. This permits the determination of the target air-fuel ratio by obtaining a first target air-fuel ratio based on the amount of intake air and a second target air-fuel ratio based on the throttle opening and then by determining a final target air-fuel ratio based on the both first and second target air-fuel ratios. In this case, it may also be considered that the amount of intake air is corrected by the throttle opening.
As have been described above, the determination of the final target air-fuel ratio on the basis of the amount of intake air and the throttle opening permits a stabilization of air-fuel artios during the constant operation as well as a highly accurate determination of air-fuel ratios. It has been found, however, that this may be encountered with a problem that a final target air-fuel ratio is caused to be deviated to a large extent during the transition period when driving states of an engine are changed. It was found that this deviation was caused by the fact that there was a difference between a response to the detection of the throttle opening and a response to the detection of the amount intake air. A delay in the response to the detection of the amount of intake air with respect to the response to the detection of the throttle opening is caused to occur because the amount of intake air has heretofore been subjected to the smoothing treatment for a prevention from the hunting or the like.